In recent years, in proportion to increases in the volume of communication information, communication in a wavelength division multiplexing transmission system in which a plurality of optical signals having different wavelengths are multiplexed to one optical fiber has been broadly utilized. In this method, an optical integrated circuit structured from a coupler which couples optical signals having a plurality of wavelengths, a branching filter which branches a plurality of optical signals in one optical fiber into different ports, or the like, in addition to a light emitting element and a light receiving element, is used.
As such a coupler, a branching filter and the like which structure an optical integrated circuit, various types of those can be used. A waveguide semiconductor device thereamong is suitable for an accumulation with other waveguide devices and passive waveguides, and is preferably used.
Further, as an example of a waveguide device, there are cases in which an MMI (Multi Mode Interference) structure is used (Patent Document 1). In an MMI type coupler/branching filter, a coupling/branching function of light waves is realized by using a multimode waveguide in which higher mode propagating is possible, and by utilizing interference among the respective modes in the waveguide. By using an MMI type structure, it is possible to reduce optical loss, and to improve the stability in manufacture.
FIG. 21 is a diagram showing a schematic structure of a waveguide portion of a conventional waveguide coupler. A multimode waveguide 101 having a mesa shape which is formed such that a core layer and a guide layer are stacked in this order is formed on a substrate 100. The multimode waveguide 101 is structured so as to be embedded with a buried layer 200. This coupler has input ports 103a and 103b formed from fundamental mode waveguides at one end of the multimode waveguide 101, and has an output port 105 formed from a fundamental mode waveguide at the other end of the multimode waveguide 101. The multimode waveguide 101 has a width wider than the input ports 103a and 103b, and the output port 105, and provides modes including multimode with respect to the waveguides.
Single mode lights respectively having peculiar wavelengths are incident into the input ports 103a and 103b, and these are guided into the multimode waveguide 101. The incident lights advance inside the multimode waveguide 101 while varying interference patterns in accordance with an advanced position, and are emitted from the output port 105 on the right side in the drawing.
In this coupler, the optical output face is a (110) plane, and a light is guided in the <110> direction. The side faces of the multimode waveguide 101 are side faces parallel to the <110> direction, and the end face at the optical output side is formed from the (110) plane, and the end face at the optical incident side is formed from the (−1-10) plane. [Patent Document 1] U.S. Pat. No. 5,640,474.